Method of vacuum investment casting

ABSTRACT

A method of making castings according to the wax elimination method, comprising the steps of mounting a wax pattern on a holding pin member, placing the pattern in a substantially centered position in a cuvette with said holding pin extending upwardly therefrom, pouring into said cuvette an investment slurry of a type setting into an air permeable, rigid, heat resistant block, allowing said slurry to set, causing said pattern and said holding pin to disappear from the mould block thus produced, placing said mould block in an opening in a vacuum chamber in such a manner that the hole left by said holding pin communicates with the atmosphere outside said vacuum chamber whilst a substantial portion of the remaining block surface is situated inside the chamber sealed off from the said atmosphere, filling liquid casting material into said hole in the mould block and subjecting said vacuum chamber to vacuum so as to cause said casting material to be sucked into the cavity left by said pattern, allowing the casting material to solidify and removing the mould block from the vacuum chamber, breaking the mould block in order to release the casting therefrom, and separating the part of the casting corresponding to said pattern from the part corresponding to the passage left by said holding pin, characterised by providing the top portion of said mould block with a radially protruding, annular flange prior to its introduction into the vacuum chamber and by inserting the mould block into the vacuum chamber opening so as to bring the said flange into sealing engagement with the edge of the opening by the movement thereagainst.

United States Patent [191 Rasmussen Dec. 25, 1973 METHOD OF VACUUM INVESTMENT CASTING [76] Inventor: Jens Ejnar Rasmussen, Langgade 3,

Randers, Denmark [22] Filed: June 17, 1971 [21] Appl. No.: 154,000

[52] US. Cl 164/35, 164/65, 164/213,

164/255, 164/376 [51] Int. Cl. B22c 9/02, B22d 27/16 [58] Field of Search 249/54; 164/61, 63,

Primary ExaminerJ. Spencer Overholser Assistant Examiner-John E. Roethel Attorney-Craig, Antonelli & Hill [57] ABSTRACT A method of making castings according to the wax elimination method, comprising the steps of mounting a wax pattern on a holding pin member, placing the pattern in a substantially centered position in a cuvette with said holding pin extending upwardly therefrom, pouring into said cuvette an investment slurry of a type setting into an air permeable, rigid, heat resistant block, allowing said slurry to set, causing said pattern and said holding pin to disappear from the mould block thus produced, placing said mould block in an opening in a vacuum chamber in such a manner that the hole left by said holding pin communicates with the atmosphere outside said vacuum chamber whilst a substantial portion of the remaining block surface is situated inside the chamber sealed off from the said atmosphere, filling liquid casting material into said hole in the mould block and subjecting said vacuum chamber to vacuum so as to cause said casting material to be sucked into the cavity left by said pattern, allowing the casting material to solidify and removing the mould block from the vacuum chamber, breaking the mould block in order to release the casting therefrom, and separating the part of the casting corresponding to said pattern from the part corresponding to the passage left by said holding pin, characterised by providing the top portion of said mould block with a radially protruding, annular flange prior to its introduction into the vacuum chamber and by inserting the mould block into the vacuum chamber opening so as to bring the said flange into sealing engagement with the edge of the opening by the movement thereagainst.

6 Claims, 3 Drawing Figures t t t s s PATENTEB DEC 2 5 I973 INVENTOR.

JENS EINAR RASPIUSSEN BY Qw mtowwlz 7 mm AT TORWEYS 31 METHQD F VACUUM HNVESTMENT CASTING The present invention relates to vacuum casting and more specifically to precision casting according to the so-called wax elimination method for producing individual castings of precious metal for dental use of for use in the art of jewellery. For making gold crowns it has been common practice to model up a wax crown on the tooth mounting the wax pattern on the end ofa narrow holding pin, placing the wax pattern in a mould, a so-called cuvette, in the middle of the mould cavity thereof, held by the said holding pin, pouring a suitable investment slurry into said mould cavity, allowing the investment to set, removing the block of investment from the mould and subjecting it to a burning treatment whereby the wax pattern as well as the holding pin are burnt away, placing the investment block in a sling, placing the necessary amount of precious metal, heated to be in a molten state, on or in a depression in the top side of the block communicating with the passage left by the burnt away holding pin, slinging the block around so as to cause the molten metal to be pressed into the mould cavity left by the burnt away wax pattern by virtue of the cenrifugal forces, allowing the molten metal to harden, breaking or smashing the investment block to pieces and taking out the casting, cutting off the metal remaining as a pin cast in the said passage, and polishing the cast metal body.

This method involves the disadvantage that the slinging is a laborious and somewhat dangerous step by which it is not always obtained that all air is driven out of the mould cavity so that blisters may occur in the final casting. It has been suggested, therefore, to avoid the slinging and instead place the investment block in a vacuum device so as to suck out the air from the cavity prior to the filling with molten metal. However, this is rather complicated because the metal shall be handled inside the evacuated space.

According to an improved method described in the U.S. Pat. No. 2,877,523 of Mar. 17, 1959, there is used a mould consisting of air permeable material of which the lower portion containing the mould cavity is placed in a cylinder in which the free space is filled out with a coarse granular material thus acting as a support for the mould. The annular space between the top end of the cylinder and the mould is filled out with an air sealing material made by pouring down a thick viscous ceramic slurry on the top surface of the said granular material whereby this slurry will stiffen and make an air tight collar around the mould. The cylinder is con nected to a vacuum source through a valve which is opened when the said collar has set, whereby the molten metal at the top of the mould is sucked down into the cavity through the said passage, the mould cavity hereby being evacuated through the permeable wall of the mould. It is then possible to work with the metal outside the vacuum space, but the method is disadvantageous in that it is difficult to mount the mould in the cylinder and to keep the assembly hot during the setting of the said sealing material, and for high quality precision castings it is normally a condition that the mould should not be cooled considerably before the casting is made.

It is the object of the present invention to provide a casting method enabling the mould to be mounted in the vacuum cylinder in a very simple manner and the casting to be made immediately thereafter, i.e., while the mould is still hot.

According to the invention there is provided a method comprising the steps of preparing a mould of air permeable material so as to obtain a mould having the desired mould cavity and mould passage as well as an annular, radially protruding flange portion adjacent the top of the mould, the height of the mould portion underneath the lower side of said flange being less than the height of the cylinder, putting this mould down into the cylinder so that the lower side of the flange rests on the top edge of the cylinder, and performing the casting operation by sucking molten metal down into the mould cavity by means of vacuum applied to the cylinder. in this manner it is not necessary to make special provisions for sealing the space between the mould and the top of the cylinder, since the necessary sealing is automatically obtained when the flange rests on the top edge of the cylinder. Therefore, the casting may be effected immediately after placing the mould in the cylinder, i.e., while the mould is still very hot. Moreover, any subsequent variations in the dimensions of the different parts due to the following temperature drop will have no effect on the efficiency of the sealing.

The mould may be made in different manners with the said flange forming an integral or disintegral portion thereof. in the latter case the mould itself may be cast as a straight cylindrical member which is thereafter immediately subsequent to or preferably prior to the heating thereof mounted in a ring member so as to be sealed thereagainst, e.g., a metal ring is pressed down over or onto a conical top portion of the mould, whereafter this assembly is mounted in the vacuum cylinder. According to a special feature of the invention, however, it is possible to directly cast the mould with the said flange as an integral part thereof, so that the flange is provided without special attention being necessary. To this end the invention also comprises a cuvette the mould cavity of which has an upper annular portion of increased diameter. By the casting of the mould the said flange will be cast automatically as an integral portion of the mould.

According to a further special feature of the invention the said cuvette is made of rubber or a similar material whereby it is easy to release the set mould from the cuvette simply by heeling over the cuvette from the mould block.

In the following the invention is described in more detail, by way of example, with reference to the accompanying drawing, in which:

FIG. 1 is a perspective view, partly in section, of a cuvette according to a preferred embodiment of the invention,

FIG. 2 is a perspective exploded view of a holding pin for a wax pattern and a carrier member therefor, and

HO. 3 is a sectional view of the mould as mounted in the vacuum cylinder.

The cuvette shown in FIG. 1 is designated 2 and is constituted by a rubber block having a lower cylindrical mould cavity or hole 4 with an upper portion 6 of enlarged diameter. Between the two hole portions 4 and 6 there is an annular horizontal surface portion 8. ln the upper edge of the cuvette there is provided two opposed notches 10.

FIG. 2 shows a holding device adapted to be placed on the cuvette shown in FIG. 1. It comprises a wire cross 12,13 provided with a central disc 14 of rubber.

The underside 16 of the disc 14 is downwardly convex or cone shaped. This lower part of the disc 14 is provided with a vertical, narrow bore 18 which is adapted to receive the upper end of a plastics pin 20, the lower end of which, in a manner known per se, is adapted to hold a wax pattern 22 secured thereto by gluing or otherwise. Of the wire pieces or pins in the wire cross the pin 13 is located in a plane above the pin 12 and has a length corresponding to the exterior diameter of the top of the cuvette 2. The lower pin 12 has an extension '12 which is usable as a handle.

In order to prepare a mould for a metal casting the pin 20 with the wax pattern 22 is inserted in the hole 18 whereafter the holding cross device is placed on the top of thecuvette 2 with the wire piece 12 resting. in the notches I and the wire piece 13 resting directly on the edge of the cuvette. In this manner it is easy to place the cross in such a position that the depending pin with the pattern 22 will be situated in the center line of the mould cavity 4. The length of the pin 20 is so adapted that the wax pattern 22 in its suspended positon will be located approximately midways of the height of the lower cavity 4, i.e., in an absolute centered position therein.

Thereafter a suitable investment slurry is poured into the cuvette until the slurry surface stands immediately underneath the cross pin 12. The slurry may consist of a mixture of quarz, plaster of paris, and waterin quite usual proportions for making an investment. When the slurry has set the cross member 12,13 with the disc 14 is retracted from the top of the pin 20 and lifted away from the cuvette. The lower side 16 of the disc 14 will hereby leave a depression in the top side of the mould block, and the top of the pin 20 will project up through this depression.

Thereafter the cast mould block is released from the cuvette 2; the cuvette being of rubber, the block is very easy to release, since the cuvette wall may simply be heeled off from the block. The mould block is then subjected to the usual burning treatment whereby the wax pattern 22 and the pin 20 are burnt away. After this treatment the mould block is as shown in a sectional view at the top of FIG. 3; it is designated24 whilst the upper, wide portion thereof, correspondinging to the cuvette section 6, is designated 28. The moulding cavity left by the wax pattern is designated 36 and the passage left by the pin 20 is designated 38. The said depression in the top side of the mould block as provided by the lower side 16 of the disc 14 is designated 32.

After the burning treatment the mould block 24 is put down into the open end of a cylinder 26 which is mounted on a tube stud 30 by means of a screw threaded bottom hole 40 with which the cylinder is screwed down onto the upper end of the stud and secured there by means of a counter nut 42 with an intermediate sealing disc 44. The upper end of the stud 30 is closed by a top plate 46 in order to secure that material particles cannot fall down into the stud, and radial bores 48 are provided in the stud wall immediately underneath the top plate 46 so as to communicate with the interior of the cylinder 26. The stud 30 is secured in a vertical position by means of an annular holding flange 50 fastened to a rigid support plate 52 by means of screws 54. The stud 30 is extended downwardly in a tube portion 56 on the end of which there is mounted a vacuum hose 58 which is connected to a vacuum source schematically indicated at 33, a manually operable valve or a remote controlled magnet valve 35 being inserted in this connection.

The mould block 24 is narrow enough to be able to pass freely down into the cylinder 26, whilst the upper flange portion 28 thereof is so broad that it will be rested against the top edge of the cylinder, as shown.

Immediately after the heating of the mould block 24 a small piece of precious metal, designated 60, is laid in the depression and heated by means of a burner 34 so as to rapidly melt. The valve 35 is opened so that the interior of the cylinder 26 is evacuated whereby air is sucked out from the mould cavity 36 and from the sprue passage 38. The molten metal 60 which may alternatively be applied to the depression 32 in an already molten condition will thereby be sucked down into the cavity 36 through the passage 38 so as to entirely fill out the cavity.

It will be appreciated that once the mould block 24 has been cast with a flange 28 having a plane and smooth lower surface as cast against the cuvette surface portion 8 it will not be necessary to provide special sealing means between the block and the cylinder, since the flange will automatically be suckedagainst the top edge of the cylinder and thus seal off the interior of the cylinder from the atmosphere. If desired, of course, the top edge surface of the cylinder as well as the lower side of the flange 28 may be slightly conical so that the block will be self-centering in the cylinder; the surface 8 of the cuvette should then be correspondingly conical.

When the cavity 36 is full and the metal has soldified the valve 35 is closed and the block 24 removed from the cylinder whereafter it is broken or smashed into pieces and the remaining metal casting treated in the usual manner.

It is within the scope of the invention to provide the mould block 24 with a flange 28 otherwise as here described, e.g. as indicated in dotted lines in the right hand side of FIG. 3 casting the mould block with a slightly protruding, conical flange which is received in an exterior ring member forming the annular member to be rested on the top edge of the cylinder 26. In the left hand side of FIG. 3 it is indicated in dotted lines that the exterior ring member may have a groove in its interior side so that it can be placed in the cuvette section 6 and then be associated with the mould block by the casting thereof. The groove, of course, may be substituted by a protruding annular flange or spaced protrusions on the interior side of the ring.

It will be apppreciated that the method according to the invention will also be perfectly usable in connection with cold castings of materials other than metal, but its advantages are most expressed in connection with hot castings where the metal may be molten directly in the depression 32.

In this disclosure reference has been made to the socalled wax elimination method which is indicative of using a model or pattern 22 of a material which is able to be burnt away when the mould block 24 is subjected to the said burning treatment. In this connection it should be emphasized that for this purpose there is a row of pattern materials available which are not exactly wax in the technical sense of this term, and of course the term wax" as used in this disclose and the appended claims should be understood in a correspondingly broad sense.

The holding pin 20 for the wax pattern may be adapted to be pulled out from the mould block 24 before the burning treatment thereof. In this case the pin should preferably be slightly conical, and the pattern should be mounted on the pin in an easily releasable manner, e.g., by means of an adhesive rather than a strong glue.

What is claimed is:

l. A method of making castings according to the wax elimination method, comprising the steps of mounting a wax pattern on a holding pin member, placing the pattern in a substantially centered position in a cuvette with said holding pin extending upwardly therefrom, pouring into said cuvette an investment slurry of a type setting into an air permeable, rigid, heat resistant block, allowing said slurry to set, causing said pattern and said holding pin to disappear from the mould block thus produced, placing said mould block in an opening in a vacuum chamber in such a manner that the hole left by said holding pin communicates with the atmo sphere outside said vacuum chamber whilst a substantial portion of the remaining block surface as situated inside the chamber sealed off from the said atmosphere, filling liquid casting material into said hole in the mould block and subjecting said vacuum chamber to vacuum so as to cause said casting material to be sucked into the cavity left by said pattern, allowing the casting material to solidify and removing the mould block from the vacuum cham-ber, breaking the mould block in order to release the casting therefrom, and separating the part of the casting corresponding to said pattern from the part corresponding to the passage left by said holding pin, characterised by providing the top portion of said mould block with a radially protruding,

annular flange prior to its introduction into the vacuum chamber and by inserting the mould block into the vacuum chamber opening so as to bring the said flange into sealing engagement with the top edge of the opening by the movement thereagainst.

2. A method according to claim 1 and comprising the step of pouring the said investment slurry into a cuvette which has an upper cylindrical portion of enlarged inner diameter, so as to cast the said flange as an integral portion of the mould block.

3. A method according to claim 2, characterised by the steps of pouring the investment slurry into a cuvette made of a rubber like material and heeling off the cuvette from the investment block after the setting thereof.

4. A method according to claim 1, comprising the step of providing the portion of the mould block extending beneath the flange with a width smaller than the width of the vacuum chamber.

5. A method according to claim 1, comprising the step of providing the portion of the mould block extending beneath the flange with a height less than the height of the vacuum chamber such that the mould block is supported in the vacuum chamber at the top edge thereof and the mould block upon application of a vacuum to the chamber is drawn downwardly into the chamber to ensure sealing engagement of the flange and the top edge of the vacuum chamber.

6. A method according to claim 1, comprising the step of subjecting the mould block to heating to cause disappearance of the wax pattern and inserting the mould block into the opening of the vacuum chamber while the mould block is in a hot condition. 

1. A method of making castings according to the wax elimination method, comprising the steps of mounting a wax pattern on a holding pin member, placing the pattern in a substantially centered position in a cuvette with said holding pin extending upwardly therefrom, pouring into said cuvette an investment slurry of a type setting into an air permeable, rigid, heat resistant block, allowing said slurry to set, causing said pattern and said holding pin to disappear from the mould block thus produced, placing said mould block in an opening in a vacuum chamber in such a manner that the hole left by said holding pin communicates with the atmosphere outside said vacuum chamber whilst a substantial portion of the remaining block surface as situated inside the chamber sealed off from the said atmosphere, filling liquid casting material into said hole in the mould block and subjecting said vacuum chamber to vacuum so as to cause said casting material to be sucked into the cavity left by said pattern, allowing the casting material to solidify and removing the mould block from the vacuum cham-ber, breaking the mould block in order to release the casting therefrom, and separating the part of the casting corresponding to said pattern from the part corresponding to the passage left by said holding pin, characterised by providing the top portion of said mould block with a radially protruding, annular flange prior to its introduction into the vacuum chamber and by inserting the mould block into the vacuum chamber opening so as to bring the said flange into sealing engagement with the top edge of the opening by the movement thereagainst.
 2. A method according to claim 1 and comprising the step of pouring the said investment slurry into a cuvette which has an upper cylindrical portion of enlarged inner diameter, so as to cast the said flange as an integral portion of the mould block.
 3. A method according to claim 2, characterised by the steps of pouring the investment slurry into a cuvette made of a rubber like material and heeling off the cuvette from the investment block after the setting thereof.
 4. A method according to claim 1, comprising the step of providing the portion of the mould block extending beneath the flange with a width smaller than the width of the vacuum chamber.
 5. A method according to claim 1, comprising the step of providing the portion of the mould block extending beneath the flange with a height less than the height of the vacuum chamber such that the mould block is supported in the vacuum chamber at the top edge thereof and the mould block upon application of a vacuum to the chamber is drawn downwardly into the chamber to ensure sealing engagement of the flange and the top edge of the vacuum chamber.
 6. A method according to claim 1, comprising the step of subjecting the mould block to heating to cause disappearance of the wax pattern and inserting the mould block into the opening of the vacuum chamber while the mould block is in a hot condition. 